Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butylenes, butadiene, and aromatics such as benzene, toluene, and xylenes. Each of these is a valuable commercial product in its own right. For instance, the olefins may be oligomerized (e.g., to form lubricant basestocks), polymerized (e.g., to form polyethylene, polypropylene, and other plastics), and/or functionalized (e.g., to form acids, alcohols, aldehydes and the like), all of which have well-known intermediate and/or end uses. One thermal cracking process is steam cracking, which involves cracking hydrocarbons in the presence of hydrogen and/or hydrogen-containing components, such as steam.
The starting feedstock for a conventional olefin production plant, as described above, has been subjected to substantial (and expensive) processing before it reaches the olefin plant. Normally, whole crude is first subjected to desalting prior to being distilled or otherwise fractionated or cracked into a plurality of parts (fractions) such as gasoline, kerosene, naphtha, gas oil (vacuum or atmospheric) and the like, including a high boiling residuum (“resid”). The resid cut typically has a boiling point of greater than about 650° F. (343° C.), at about atmospheric pressure. After desalting and removal of the resid fractions, conventionally, any of these fractions, other than the 650° F.+ (343° C.+) resid, may be passed to a steam cracker or olefin production plant as the feedstock for that plant.
Typically in steam cracking, a hydrocarbon feedstock for steam cracking, such as naphtha, gas oil, or other non-resid containing fractions of whole crude oil, which may be obtained, for instance, by distilling or otherwise fractionating whole crude oil, is usually mixed with steam and introduced to a steam cracker. Conventional steam cracking utilizes a pyrolysis furnace that generally has two main sections: a convection section and a radiant section. In the conventional pyrolysis furnace, the hydrocarbon feedstock enters the less severe convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is heated and vaporized by indirect contact with hot flue gas from the radiant section and optionally by direct contact with steam. The vaporized feedstock and steam mixture (if present) is then introduced through crossover piping into the radiant section where it is quickly heated, at pressures typically ranging from about 10 to about 30 psig, to a severe hydrocarbon cracking temperature, such as in the range of from about 1450° F. (788° C.) to about 1550° F. (843° C.), to provide thorough thermal cracking of the feedstream. The resulting products, comprising olefins, leave the pyrolysis furnace for further downstream separation and processing.
After cracking, the effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety, e.g., saturated, monounsaturated, and polyunsaturated, and can be aliphatic and/or aromatic, as well as significant amounts of molecular hydrogen. The cracked product is then further processed such as in the olefin production plant to produce, as products of the plant, the various separate individual streams of high purity mentioned above, i.e., hydrogen, the light olefins ethylene, propylene, butenes, and aromatic compounds, as well as other products such as pyrolysis gasoline.
As worldwide demand for light olefins increases and the availability of favorable crude sources is depleted, it becomes necessary to utilize heavier crudes (i.e., those having higher proportions of resid), which requires increased capital investments to process and handle the refining byproducts and purchase the higher grade feedstocks. It is highly desirable to have processes that can take lower cost, heavier crudes, and produce a more favorable product mix of light olefins, more efficiently.
It has previously been proposed to upgrade certain crude fractions other than resid, prior to steam cracking, by first hydroprocessing the feed. For instance, U.S. Pat. Nos. 3,855,113 and 6,190,533 are directed to an integrated process comprising passing the feed to a hydroprocessing zone followed by a steam cracking zone. In neither case, however, is whole crude or a fraction comprising resid passed directly to the hydroprocessing zone or to the steam cracker. See also GB 2,071,133 and Erdoel & Kohle, Erdgas, Petrochemie (1981), 34(1), 443-6. Other references of interest include U.S. Pat. Nos. 3,855,113; 4,057,490; 4,179,355; and 6,743,961.
Conventional resid hydroprocessing or “residfining” is a known process for upgrading a portion of the resid containing crude fraction. The hydrogenated liquid and vapor products (but not the resid products) from residfining are typically fractionated into more valuable streams, e.g., fuel oil, diesel, heating oil, jet, kerosene, gasoline, LPG, and fuel gas. Each of these is useful per se as fuels and/or as intermediates for the production of, for instance, petrochemicals. By way of example, fuel oil may also be cracked to produce the lighter boiling fuels, such as gasoline, LPG, and fuel gas and/or the petrochemicals ethylene, propylene, and butanes. The resid fraction is typically a low-value product. However, subsequent to hydroprocessing and before or during the further distillation of the resid stream, conversion, deasphalting, or other processing may be performed on the resid containing crude fraction.
U.S. Pat. No. 3,898,299 discloses a process for removing the resid fractions and producing olefins from the non-resid, lower boiling point hydrocarbons. Atmospheric resid from distillation is hydroprocessed and the liquid hydroprocessor effluent is “fed in the presence of steam directly to the pyrolysis zone wherein unvapourized feedstock is removed as a residue fraction in a separation zone prior to entry of the vapourised distillate fraction into that region of the pyrolysis zone maintained under conditions which effect thermal cracking.” However, the '299 reference only teaches conventional hydroprocessing and thermal steam cracking of the non-resid-containing overhead stream and does not properly suggested or teach how to use the resid-containing effluent from a resid hydroprocessing unit as a feed for a steam cracker. The '299 patent requires removal of the 650° F.+ (343° C.+) boiling point fractions, from the treated hydroprocessor effluent. Only the distillate fractions are processed to olefins. Those skilled in the art are well aware of the practical difficulties involving equipment fouling of conventional equipment of the U.S. Pat. No. 3,898,299 patent, for steam cracking resid-containing feeds. Resid hydroprocessing is a known process for upgrading resid to fuels such as fuel oil, diesel, heating oil, jet, kerosene, gasoline, LPG, and fuel gas. Each of these is useful as fuels and/or as intermediates for the production of, for instance, petrochemicals.
WO 2004/005431 discloses a process for steam cracking certain feedstocks comprising resid, whereby a substantial unconverted liquid resid fraction is removed prior to steam cracking. The '5431 invention does not disclose or teach hydrogenation as a process useful for upgrading heavy, sour, crude oil and resid feedstocks, including the resid fractions, such that the whole crude, including resid fractions, may be steam cracked and converted to petrochemicals. Heavy, sour feedstocks do not contain high concentrations of the linear paraffins, which are known to make the highest quality steam cracker feedstocks. The atmospheric and vacuum resid fractions of crude oils containing >2.0 wt % sulfur almost always have a hydrogen content <12.5 wt % and typically they have a hydrogen content of <11.0 wt %. It is well known that conventional resid hydroprocessing produces product highly prone to fouling.
Other patents of interest related to cracking heavy feeds include U.S. Pat. No. 4,257,871, to Wernicke; U.S. Pat. No. 4,065,379, to Soonawala; U.S. Pat. No. 4,180,453, to Franck; and U.S. Pat. No. 4,210,520, to Wernicke. There remains in the art, means and processes for economical processing of heavy, resid containing whole crudes, and resid containing hydrocarbon fractions thereof, for the production of olefins, aromatics, and other valuable petrochemical products. All known art previous to this invention, has deficiencies, shortcomings, or undesirable aspects.